Various attempts have been made to produce particulate synthetic polymers derived from expanded polymers or polymer solutions for use in the paper-making industry as modifiers to improve or modify the properties of the fibrous paper products. These particulate synthetic polymer products of the prior art have met with limited commercial success either because of cost or incompatibility with standard paper-making processes and formulations. Generally, the use of a particulate polymer in a fibrous paper formulation results in decreased tensile and tear strength properties of the modified product.
One attempt to use a particulate synthetic polymer as a modifier in a paper product is described in U.S. Pat. No. 3,038,867. A "semi rigid" urea-formaldehyde foam is prepared and disintegrated. In order to process a mixture of this disintegrated aminoplast resin and wood pulp on paper-making equipment it was necessary to de-aerate the mixture. No physical properties on tensile or tear strength are reported. However, the required de-aeration step indicates that the disintegrated foam contained sufficient cell structure to entrap air and cause floatation of the disintegrated foam from the wood pulp component of the paper-making formulation which in turn caused the processing problem reported in Example 1 of the patent.
A second attempt to produce a particulate polymeric modifier is described in U.S. Pat. No. 3,597,312. A particulate or fibrous polystyrene is prepared according to this patent by precipitating a solution of polystyrene in a non-solvent such as pentane. The resulting precipitate is recovered and extracted with non-solvent to remove entrapped solvent; the extraction step is then followed by drying of the polymer to remove the non-solvent. This process is costly and to some extent dangerous because it involves the use and recovery of large quantities of potentially explosive and relatively expensive organic solvents.
Another attempt to produce a particulate foamed urea-formaldehyde resin suitable for use in paper making is described in U.S. Pat. No. 3,164,559. At this point the art had come to recognize that granular particulates exhibited little or no mechanical binding attraction for the fibrous substances with which they were to be combined. The patent describes an attempt to use a flexible urea-formaldehyde resin as the starting material for making a paper additive. The inventors solved the problem of grinding such a flexible material by compressing it under temperature and pressure until such densification eliminated the resilient property of the polymer which interfered with the grinding operation. The product of this process is deficient in that its addition to a paper-making formulation caused a major reduction in tensile and breaking strength.